FIG. 1A is a circuit diagram of a conventional grounded-gate N-channel metal-oxide-semiconductor field-effect transistor (ggNMOS) protection device, commonly used in complementary metal-oxide-semiconductor (CMOS) technology. It comprises an NMOS transistor whose source and gate are connected to the ground. The substrate of the transistor, typically common to all transistors of the circuit, is also connected to ground. The drain of the transistor forms a hot terminal H of the protection device, the cold terminal being the ground. The device 10 generally comprises a terminal connected to ground and one or more terminals sensitive to electrostatic discharge. Each of these sensitive terminals may be connected to a terminal H of a dedicated protection device.
FIG. 1B shows a ggNMOS device. The operation of the device is determined by the parasitic NPN bipolar transistor of the MOS transistor. This bipolar transistor Q has its emitter connected to ground and its collector connected to the terminal H. The base of transistor Q is formed by the substrate of the MOS transistor. The area of the substrate useful to the transistor Q is at a given distance from an ohmic contact connecting the substrate to ground, so that there is a resistance Rd, referred to as “depolarization resistance,” between the base of transistor Q and ground. Moreover, the base and collector are connected through a capacitor Cdb representing the drain-substrate capacitance of the MOS transistor.
This type of protection device has two operating modes, a static mode and a dynamic mode. In static mode, when the voltage on terminal H is sufficiently high, the collector-base junction of transistor Q assumes an avalanche state. The base-emitter voltage exceeds the junction threshold and the transistor becomes conductive, short-circuiting (i.e. shorting) the terminal H to ground. In dynamic mode, when the voltage on terminal H increases fast enough, the capacitor Cdb has no time to charge through resistor Rd. The base-emitter voltage reproduces the transition of the voltage at terminal H, such that the junction threshold is exceeded and the transistor turns on to counter the phenomenon.
This type of protection device is used with CMOS technology, since it is designed from a MOS transistor without violating the available design rules and without requiring the creation of a dedicated component. The values of capacitor Cdb and resistance Rd are moreover controllable to adjust the desired dynamic characteristics.
Electro-optical components made of semiconductor materials may also require protection against electrostatic discharge. Optical and electro-optical components that are integrated together are simple compared to components that are integrated with CMOS technology, whereby dedicated manufacturing techniques have been developed for optical components that are inexpensive compared to CMOS technologies.